1. Field of the Invention
The present invention relates to a process of producing a fluorophosphate glass and more particularly to a process of producing a fluorophosphate glass having an index of refraction n.sub.e of more than 1.57 and an Abbe number .nu..sub.e of less than 70, said glass having furthermore a relatively high positive anomalous partial dispersion +.DELTA..nu..sub.e, and to such a glass.
2. Description of the Prior Art
It is known to produce, by melting, glasses being based on metaphosphates of Group II and Group III of the periodic system with the addition of simple and/or complex inorganic fluorides, said glasses having a positive anomalous partial dispersion.
Thus there is known from U.S. Pat. No. 3,492,136 a process for the manufacture of a fluorophosphate glass the starting mixture of which, in addition to calcium and/or aluminum metaphosphate, also contains alkaline earth metal fluorides and one of the compounds potassium meta-arsenate, potassium heptafluoro tantalate, or, respectively, potassium hexafluoro titanate. The glass produced by this known process have indices of refraction n.sub.e between 1.53 and 1.55. Such indices must be considered relatively high for this type of glass. Furthermore, U.S. Pat. No. 3,671,276 discloses an improvement in the process described in U.S. Pat. No. 3,492,136 by which fluorophosphate glasses having indices of refraction n.sub.e between 1.50 and 1.56 can be produced.
Thus according to these known processes it was possible to produce optical fluorophosphate glasses of medium to low dispersion -- corresponding to a medium to high Abbe number .nu..sub.e -- and of indices of refraction n.sub.e between 1.53 and 1.56. Such glasses, furthermore, exhibit a positive anomalous partial dispersion +.DELTA..nu..sub.e.
It is, however, of interest with respect to certain fields of use in the optical industry to provide glasses of even higher refractive values n.sub.e &gt;1.57 and of optimum dispersion, but which meet the requirements of the calculating optician, namely requiring that such glasses also retain, or at least only slightly reduce, their positive anomalous partial dispersion values. Such glasses, heretofore, have not been produced.